1. Field of the Invention
The present invention relates to a liquid crystal device used by a single person or a plurality of people, such as a wordprocessor, a personal computer, or a mobile information terminal, a photoinitiator suitable for such a liquid crystal display device, a photopolymerizable resin composition containing such a photoinitiator, and a polymer formed from such a composition. In this specification, a liquid crystal display device will be referred to as an "LCD device".
2. Description of the Related Art
Conventionally, the following LCD devices using a composite of a liquid crystal material and a polymer are known.
Japanese National Phase PCT Laid-Open Publication No. 58-501631, for example, describes a polymer dispersed liquid crystal display device (hereinafter, referred to as a "PDLC display device"). The PDLC display device includes liquid crystal domains each surrounded by a polymer matrix. When no voltage is applied to the liquid crystal material, the refractive indices of the liquid crystal material and the polymer are not matched, resulting in a scattering state. When a voltage is applied to the liquid crystal material, the refractive index of the liquid crystal material changes so as to be matched with that of the polymer, resulting in a transparent state.
Japanese National Phase PCT Laid-Open Publication No. 61-502128, for example, describes an LCD device in which a mixture of the liquid crystal material and the photopolymerizable resin is irradiated with ultraviolet rays, resulting in three-dimensional phase separation into the liquid crystal material and a polymer.
The above-described LCD devices perform display by electrically controlling the liquid crystal material to be in the scattering state or the transparent state. The liquid crystal domains do not have any spatial regularity.
In order to provide a regularity to liquid crystal domains formed by irradiating a mixture of a liquid crystal material and a photopolymerizable resin with ultraviolet rays, methods of irradiation of ultraviolet rays through a photomask as described below have been disclosed.
Japanese Laid-Open Publication No. 1-269922 describes the following method. A mixture of a liquid crystal material and a photopolymerizable resin is subjected to a first exposure to light through a photomask. After the photomask is removed, the mixture is subjected to a second exposure with ultraviolet rays, so that an area covered by the photomask during the first exposure is irradiated with ultraviolet rays. Thus, areas having different electrooptical characteristics are formed. An LCD device produced by this method basically performs in a scattering mode.
Japanese Laid-Open Publication No. 5-257135 describes an LCD device produced by the following method. An alignment layer having an alignment restricting force is provided on each of two substrates. Into a gap between the two substrates, a mixture of a liquid crystal material and a photopolymerizable resin is injected. The mixture of the liquid crystal material and the photopolymerizable resin is irradiated with ultraviolet rays through a photomask. The LCD device produced by this method is used for static driving, by which liquid crystal domains are patterned by a control performed outside the cell, utilizing that an area covered by the photomask and an area not covered by the photomask have different threshold characteristics.
As an attempt to improve the viewing angle characteristic of the LCD devices, use of a composite of a liquid crystal material and a polymer has been proposed. It is necessary that liquid crystal molecules are oriented in at least three directions in a pixel area in order to improve the viewing angle characteristic.
With reference to FIGS. 1A and 1B, the viewing angle characteristic of a wide viewing angle mode LCD device will be described.
FIG. 1A schematically shows the relationship between a change in the orientation of liquid crystal molecules and the viewing angle characteristic in accordance with application of a voltage regarding a wide viewing angle mode LCD device 10. FIG. 1B schematically shows such relationship regarding a conventional twisted nematic (TN) mode LCD device. In both FIGS. 1A and 1B, part (a) illustrates the state where no voltage is applied, part (b) illustrates an intermediate state where a voltage is applied to a certain degree less than full, and part (c) illustrates the state where the voltage is applied to a full degree.
As shown in FIG. 1A, the LCD device 10 includes plates 1 and 2. A liquid crystal layer interposed between the plates 1 and 2 includes a polymer wall 7 surrounding a liquid crystal domain 8. Liquid crystal molecules 9 included in the liquid crystal domain 8 are oriented symmetrically with respect to an axis 6. Accordingly, in the intermediate state shown in part (b), the apparent refractive indices of the liquid crystal molecules 9 when seen in directions A and B are averaged to be equal. As a result, the viewing angle characteristic is improved compared to the TN mode shown in FIG. 1B.
In the conventional TN mode LCD device shown in FIG. 1B, the liquid crystal molecules have only one orientation direction in the intermediate state shown in part (b). Accordingly, display characteristics such as the levels of brightness and the apparent refractive indices of the liquid crystal molecules when seen in directions A and B are different. As a result, the viewing angle characteristic is inferior to that of the LCD device 10.
The following LCD devices have been disclosed as wide viewing angle mode LCD devices.
Japanese Laid-Open Publication Nos. 4-338923 and 4-212928 disclose a wide viewing angle mode LCD apparatus produced by combining the above-described PDLC display device and polarizers having polarizing axes perpendicular to each other.
Japanese Laid-Open Publication No. 5-27242 discloses a method for improving the viewing angle characteristic of a non-scattering mode LCD display device using polarizers. According to the method, a mixture of a liquid crystal material and a photopolymerizable resin is phase-separated, thereby forming a liquid crystal layer formed of a composite of the liquid crystal material and a polymer. Due to such a method, the orientation of liquid crystal domains become random by the resultant polymer. In other words, the liquid crystal molecules in different domains rise in different orientation directions when a voltage is applied. As a result, the transmittance of the liquid crystal molecules becomes equal when seen in a plurality of directions, thus improving the viewing angle characteristic in a half-tone display.
The present inventors disclose the following LCD device in Japanese Laid-Open Publication No. 6-301015. The LCD device is produced by controlling the amount of light using a photomask or the like during photopolymerization, so that the liquid crystal molecules are oriented in an axially symmetrical state in a pixel area (e.g., spiral orientation). The liquid crystal molecules oriented in the axially symmetrical state are voltage-controlled to open and close in a manner to give the impression of an umbrella opening and closing. Thus, the viewing angle characteristic is significantly improved.
For producing an LCD device including a polymer wall formed by irradiating a mixture of a liquid crystal material and a photopolymerizable resin with light, Irgacure651 or Irgacure184 (produced by Ciba-Geigy Corporation) has been conventionally used as a photoinitiator.
A photoinitiator is indispensable to polymerize (i.e., cure) the above-described photopolymerizable resin. A generally used photoinitiator functions in the following manner. Molecules of the photoinitiator are cleaved by light irradiation to be radicals, which causes radical polymerization reaction with a photopolymerizable resin (monomer). Thus, the photopolymerizable resin is polymerized (i.e., cured).
However, the above-described photoinitiators have a relatively small molecular weight and are relatively highly volatile. In the case where a mixture of a liquid crystal material, a photopolymerizable resin and such a photoinitiator is pressure-reduced and defoamed and then injected into a liquid crystal panel, the photoinitiator is significantly vaporized by the pressure-reduction and thus polymerization reaction does not proceed sufficiently. As a result, the following disadvantages are generated. The display characteristics of the resultant LCD device are deteriorated. The voltage retaining ratio of the LCD device is reduced, the specific resistance of the mixture of the liquid crystal material, the photopolymerizable resin and the photoinitiator is lowered, and images unnecessarily remain on the screen, resulting in deterioration in the display quality.
In the following type of LCD devices, photopolymerization is performed in order to control the orientation of the liquid crystal molecules: (1) an LCD device in which the wide viewing angle is realized by controlling the orientation of the liquid crystal molecules in a pixel area; and (2) an LCD device including liquid crystal domains substantially surrounded by a polymer wall, in which the orientation of the liquid crystal molecules is changed in correspondence with an externally applied voltage, and a change in the polarization state of transmitted light caused by the change in the orientation is utilized for display (e.g., TN, STN (super twisted nematic), ECB (electrically controlled birefringence), FLC (ferroelectric liquid crystal), and axially symmetrical orientation mode LCD devices). Accordingly, in the case when a polymerization system contains impurities or when a reactive substance or photoinitiator which is likely to disturb the orientation of the liquid crystal molecules remains in the liquid crystal layer, undesirable display is likely to occur.